Ab763 tremolo frequency range

[Bombacaototal]

Hi all

I am just wondering if anyone knows the phase shift oscillator frequency range of a stock ab763 blackface optocoupler circuit (like a deluxe reverb)? I found different values in different sources so just wondering what is the actual correct one

What I've seen:
(I) 0.08 to 10 hz
(II) 2 to 20 hz
(III) 3 to 10 hz

Thanks a lot

[Stevem]

There is no right or wrong, just what you want or the customer wants.
In my 40 plus years of doing this 99 % of the people want a slower Trem then how the amp is made.
Most folks are happy with just doubling the value of one of the 3 caps, but I had one guy who wanted it so slow that if a did not have a really strong tube in the amp for that function then the oscillator eould not even start up!

[Bombacaototal]

Thanks for the reply Stevem! I guess the stock original amp come with .08 to 10, right

[sluckey]

3Hz to 10Hz is typical for the AB763 circuit. I've never seen a trem circuit that goes down to 0.8Hz.

[Bombacaototal]

3Hz to 10Hz is typical for the AB763 circuit. I've never seen a trem circuit that goes down to 0.8Hz.

Thanks a lot for clarifying!

[Bombacaototal]

3Hz to 10Hz is typical for the AB763 circuit. I've never seen a trem circuit that goes down to 0.8Hz.

I was doing some calculations and found the following:

The cathode bypass capacitor of 22uF 25v should give the slowest speed of 2.55 Hz. Does it make any difference in using a 25uF here?

The Phase shift oscillator uses 2 x .01 caps and 1 x .022 cap. I read that by replacing all 3 caps of the oscillator circuit to .022 values would slow the LFO max speed.

But according to my calculations all 3 caps at .022 with the stock 1M resistors will make the max speed around 4.5 Hz vs the stock 10 Hz. This would make the LFO really slow, maybe too slow? Or are my calculations not correct

If the oscillation at the highest speed settings disappears I read one can replace the cathode resistor with an LED to eliminate the need for the large bypass capacitor, and obtain maximum gain and minimum output impedance at all frequencies in one fell swoop.

Has anyone tried the .022s with and without the LED? What was the best result between both?

[TUBEDUDE]

The problem is the range is too limited by this design. for a wider range, a pedal, or a transistor oscillator is required.

[Bombacaototal]

The problem is the range is too limited by this design. for a wider range, a pedal, or a transistor oscillator is required.

Thanks for the reply! I did all the calculations with 200k resistors instead of 1M and this would give a much better range to fine tune. What I don't know is the consequence of using smaller resistors and higher caps. Will the caps take longer to charge and hence it may affect the start of the effect?

Another curiosity is if anyone tried a 100K with 820 instead of 220k (load resistor) and 2.7k (cathode resistor) on the ocillator tube

[pdf64]

See http://www.aikenamps.com/index.php/desi ... o-circuits

[Bombacaototal]

See http://www.aikenamps.com/index.php/desi ... o-circuits

Thanks for the reply! This was indeed my source for the calculations. What is not clear to me is soundwise the difference between 100K with 820 vs 220K with 2.7K, and the consequence of using smaller resistors.

Because if you follow aiken calculations with Fender values it would suggest resistors of 240K but for some reason fender preferred the 1M

[sluckey]

That phase shift oscillator is a finicky circuit. It needs a strong tube with lots of gain (reason for the 220K plate resistor rather than 100K). And it also needs to be operated with a big B+, screen node in the case of the AB763.

Using a 100K plate resistor will lower the gain of the circuit and the oscillator may fail to oscillate all together, or drop out on the high and low end of the frequency range.

[Bombacaototal]

That phase shift oscillator is a finicky circuit. It needs a strong tube with lots of gain (reason for the 220K plate resistor rather than 100K). And it also needs to be operated with a big B+, screen node in the case of the AB763.

Using a 100K plate resistor will lower the gain of the circuit and the oscillator may fail to oscillate all together, or drop out on the high and low end of the frequency range.

Thanks for the detailed explanation. I understand it now

[pdf64]

My guess for the choice of 2k7 and 220k cathode and plate resistors in the Fender oscillator were to avoid the need for a high value cathode bypass cap / allow the 25uF 25V used elsewhere in the design for cathode bypass.
My understanding is that ecaps were expensive back then.

[Bombacaototal]

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